Based on your questions I strongly suggest that you look for a book on e-commerce or something that will explain the basic concepts of how these things are generally put together.
It sounds like this isn't something you've done before and there are a lot of parts you need to get right.
benjamin curtis' book, The Money Train could be a good start: <http://www.agilewebdevelopment.com/book>
cheers, jean-pierre
Substruct, the first and only released Rails open source shopping cart (sorry had to plug it 
- http://dev.subimage.com/projects/substruct) uses the Payment gem, which hooks into Authorize.net.
Although I’ve been looking for better alternatives that support things like recurring billing. Might just add that myself, who knows.
Regards…
Im just implementing an active merchant processor
Im probably going to write a processor for payment too
As long as you have good documentation from the processor or a perl Business::Online Payment processor module, It should be fairly easy to make your own.
Payment is essentially the same as perl’s Business::OnlinePayment
it seems active merchant doesn’t support cheques , at least there is no support yet
I would like to see this all DRY’d up , that is active merchant using payment and all the code that is
I personally seem to see too many plugins ,engines , regular ruby libraries and whatever that all do the same thing